Dynamics of Hydration in Vanadia–Titania Catalysts at Low Loading

ARTICLE pubs.acs.org/JPCC

Dynamics of Hydration in Vanadia
Titania Catalysts at Low Loading: A Theoretical and Experimental Study Anna E. Lewandowska,†,# M
onica Calatayud,‡,§,∞ Frederik Tielens,||,^ and Miguel A. Ba~nares*,† †

Catalytic Spectroscopy Laboratory, Institute of Catalysis and Petroleum Chemistry, CSIC, E-28049-Madrid, Spain, UPMC Univ Paris 06, UMR 7616, Laboratoire de Chimie Theorique, Institut Universitaire de France, F-75005 Paris, France § CNRS, UMR 7616, Laboratoire de Chimie Theorique, F-75005, Paris, France ∞ Institut Universitaire de France Laboratoire de Reactivite de Surface, UPMC Univ Paris 6, 4 Place Jussieu, 75252 Paris Cedex 05, France ^ Laboratoire de Reactivite de Surface, CNRS, UMR 7197, 4 Place Jussieu, 75252 Paris Cedex 05, France

)

‡

bS Supporting Information ABSTRACT: The hydration process of dehydrated vanadia
titania catalysts at low loading is investigated using periodic DFT calculations. We focus on the early stages of the hydration process of the vanadia
titania in order to shed light onto the structural and dynamical changes occurring at the molecular level. Hydration is modeled by addition of successive water molecules to the dehydrated models. Special attention is paid to the V
OH bond formation and the transformation between different surface species. It is found that at low vanadia coverage the predominant surface species are OV(OH)O2 monomers with high affinity for water. Interestingly, OVO3 pyramids are stable only under severe dehydrating conditions, and hydroxylated species are expected to be present even at low water content. While low water content leads to water dissociation and supports hydration, higher content leads to a dynamic equilibrium between hydrated vanadia surface species. Interconversion between different surface species is fast and depends on the water coverage, through a fast hydrogen transfer mechanism. Leaching of OV(OH)3 species is observed in the case of high water content. The number of adsorbed water molecules depends on temperature, but even at high temperature, water adsorption is preferred, which is relevant to the state of titania-supported catalysts during reaction conditions in which water is fed or generated during reaction. Calculated harmonic vibrations are provided for the most stable surface species; their redshift upon coordination with water and their blueshift upon progressive dehydration are experimentally confirmed by in situ Raman spectra in dry and humid air at increasing temperatures.

1. INTRODUCTION Supported metal oxide catalysts are widely used in the chemical, petrochemical, and environmental catalytic processes.1 Among these, the titania-supported vanadium oxide materials exhibit excellent catalytic properties for several selective oxidation processes, such as o-xylene oxidation to phthalic anhydride,2
4 methanol oxidation,5
7 the oxidation of toluene to benzoic acid,8,9 and the selective catalytic reduction of NO (DeNOx process).10
12 The catalyst efficiency is related to the vanadium loading on the support, TiO2-anatase. Vanadium oxide dispersed on a titania surface forms monomeric and/or polymeric species depending on the vanadia coverage. In addition, hydration has proven to have an impact on the catalytic processes. Thus, water vapor addition to reactant stream has a promoting effect on the catalytic properties of the vanadia
titania system in the selective oxidation of hydrocarbons. For instance, steam accelerates the formation of pyridine-3-carbaldehyde intermediate and subsequent formation of nicotinic acid in the oxidation of 3-picoline. Water increases the vanadia
titania catalyst activity and selectivity.13 The presence r 2011 American Chemical Society

of water vapor in toluene oxidation provides a rise in the catalyst activity and enhances the selectivity to benzoic acid.14 The promoting role of water is related to the formation of V-OH hydroxyl groups and generation of mobile protons. Since water is the coproduct in the oxidation reactions and selective catalytic reduction, and since it acts as a promoter in some of the selective oxidation reactions, it is critical to understand the interaction of water with the catalyst surface. The knowledge of water impact on the structure of surface vanadium oxide species, the vanadium coordination sphere, the ratio between polymeric and monomeric species, and their domain size is still unknown and needs a systematic study. The higher stability of the long-lived industrial vanadia
titania catalyst, compared to the other vanadia-supported systems, generates an interest to widen the knowledge on the transformations Received: May 21, 2011 Revised: October 28, 2011 Published: October 28, 2011 24133

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The Journal of Physical Chemistry C taking place at the atomic level. In particular, the role of water on the structure of surface vanadium oxide species and the evolution between polymeric and monomeric species during the hydration process are a matter of discussion. Elementary steps in the hydration processes can be rationalized using theoretical calculations. The location of the surface hydroxyl groups and their interaction with active sites have attracted attention in the last years from ab initio insights. In one group of investigations,15,16 the catalyst models are built to mimic the experimental grafting process. The monomeric vanadium species are obtained via the condensation reaction of a model VO(OH)3 precursor molecule with adjacent OH groups of the hydroxylated (001) TiO2-anatase surface. Thermodynamically stable structures of VOx species depended on the V-coverage. Tetrahedrally coordinated VOx species showed favorable formation energy values at low coverage (θ < 0.5). They contained hydroxo-monovanadate, VO(OH), pyrovanadate, VO(OH)
O
VO(OH), and divanadate, V(O)
O
V(O), moieties. The hydroxo-monovanadate species were expected to be stable only if they were isolated. A similar construction model was used by Islam et al.17 to calculate the grafting process of the VO(OH)3 entity on the hydroxylated amorphous silica surface. It was found that the most stable structures involved one vanadyl group together with n(V
O
Si) bonds. The stability of the mono-, di-, and trigrafted species on the silica surface was calculated using the atomistic thermodynamics approach giving a more precise picture of the temperature and pressure influence on the structure stability of surface vanadium species related to the spectroscopic measurements. The composition of the surface depended on the thermodynamic conditions. The pyramidal OV(O
Si)3 species was stabilized in dehydrated conditions (high temperatures, low water partial pressure), while the hydroxylated OV(OH)(O
Si)2 and OV(OH)2(O
Si) species were predominant in hydrated conditions (low temperatures, high water pressure). The coexistence of these species was expected under certain conditions. Grybos and Witko18 used thermodynamical analyses to study the stability of surface vanadium structures on the (001)TiO2 system. The most stable structure was found to be V(OH)5 for a wide range of temperatures at atmospheric pressure. The surface hydroxyl groups (V
OH) were easily removed at elevated temperature under ultrahigh vacuum (UHV) conditions. The main conclusion of that work is the wide variety of vanadia species existing or coexisting on the catalyst surface depending on external conditions (pressure and temperature). In other studies,19,20 attention is focused on the microsolvation process, i.e., the successive water adsorption on V2O5 cluster. Adsorption took place on the uncoordinated vanadium center to reach a tetrahedral environment on this atom. Dissociative adsorption of water was more favorable than molecular at the first stages. New V—OH bonds formed as a result of H2O dissociation. Two VO(OH)3 monomers and molecular water were stabilized by hydrogen bonds upon adsorption of four molecules. The support stabilization effect on VO(OH)3 monomeric species was investigated by periodic DFT. The model was built by anchoring the OV(OH)3 unit onto the (100) titania anatase surface. The study of hydrogen distribution revealed unfavorable adsorption energy when the hydrogen atoms were exclusively placed on the support. The most stable conformation presented one hydrogen on the support and two on the monomer, forming hydroxylated OV(OH)2(O—Ti) species. The hydroxo-monovanadate OV(OH)(O
Ti)2 species was found to be less stable. It was found that a VdO group was present in every stable system. Regarding

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reactivity, ab initio calculations on a pyramid vanadia/titania model show that hydration determines a change in the adsorption mode and strength of methanol together with a slight increase in the barrier for its transformation to formaldehyde.21,22 At low vanadia coverage, the interaction of water with the support may be important due to the area exposed. The natural crystals and synthesized nanocrystals of anatase support exhibit a truncated square bipyramid shape where (101) planes form the majority of the total exposed surface, while the remaining part consists of (001) titania surface.23 Dehydrated surfaces exhibit different Lewis basic sites (unsaturated O atoms) and Lewis acid sites (unsaturated Ti atoms) concentration depending on the crystallographic planes. The interaction of water with the TiO2 surface has been studied for (101), (001), (100), and (110) titania anatase surfaces by DFT calculation.24
26 The adsorption mode, molecular or dissociated, depends on the nature of the exposed plane. Thus, for the most stable (101) plane, water remains molecular, while more reactive planes like (001) surface-stabilized dissociated moieties dominate, especially at low water coverage; at higher coverage, both molecular and dissociated forms coexist. The surface water coverage is stable at 3.5 OH nm
2 between 400 and 680 K for atmospheric pressure. The coverage decreases to 1.7 OH nm
2 as temperature rises to 840 K. The (001) titania anatase surface reaches a dehydrated state above 840 K.25 In this work, we investigate the hydration process of dehydrated vanadia
titania catalysts at low loading using periodic DFT calculation with large slab model constructed on the basis of our previous studies.19,27 The V
OH bond formation and its transformation between monomeric species are mimicked by successive water molecule additions to the models. It reflects the early stage of the hydration process of the vanadia
titania surface. The dynamic transformations between different surface vanadium oxide species at given temperatures are modeled by molecular dynamics. The stability of selected conformations is analyzed by total energy optimization. We use atomistic thermodynamics to combine the DFT calculation at 0 K with temperature and pressure conditions used in the spectroscopic measurements. This allows us to estimate the predominance of the surface species as a function of the temperature and hydration conditions. The effect on vibrational frequencies is calculated and compared with Raman spectra at different temperature and with controlled humidity.

2. METHODOLOGY The VASP package28
30 was used to perform total energy and molecular dynamics simulations. The Perdew
Becke
Erzernhof functional PBE31 has been used as implemented in the code.32 The core electrons are represented by pseudopotentials generated by the PAW (projector augmented wave) method. The valence electrons (V, 3p64s23d3; Ti, 4s23d2; O, 2s22p4; H, 1s1) are described by plane-wave basis sets with a cutoff of 250 eV. Due to the size of the system, only the gamma-point is considered in the Brillouin zone. For geometry optimization, the conjugate gradient method was employed. All atoms were allowed to relax until the total energy difference was below 1 meV. The model consists of a unit cell of dimension 15.4  15.4  25 Å3. The support is a 4  4 anatase (001) five TiO2 units thick slab, on top of which V2O5 and water are deposited. The bare vanadia/titania slab contains 247 atoms. Water is added by including consecutively one to four water molecules to the bare system; for comparison purposes, higher water content is modeled 24134

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Scheme 1. Structure of 4-Coordinated (Tx) and 5-Coordinated (Px) Monomeric Surface Vanadium Oxide Speciesa

a

White small balls are Ti atoms forming a part of the (001) TiO2 anatase surface. Green balls are V atoms, red balls are O atoms, and yellow balls are H atoms.

by including eight water molecules. A vacuum of 10 Å prevents interaction with consecutive slabs. Molecular dynamics (MD) calculations were performed using the microcanonical ensemble (constant energy molecular dynamics). The calculated Hellmann
Feynman forces serve as acceleration acting on 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: Starting at 800 K, a first run (1 fs step, 1 ps) was done. The final structure and velocities were used as a starting point at 500 K (1 fs step, 1 ps), and the same procedure was repeated at 300 K. The runs were analyzed as regards the surface species present (monomers, dimers, with different V coordinations) and their dynamics (surface mobility, hydrogen transfer, desorption). More than 20 MD runs were analyzed considering several starting geometries. Finally, the most representative structures were fully optimized at 0 K in order to characterize their stability. Harmonic vibrational frequency calculations for the most stable models have been performed. Second derivatives of the energy are obtained by finite differences for atom displacements of 0.02 Å. Then, the Hessian matrix is constructed and diagonalized to give the frequencies (eigenvalues) and modes (eigenvectors). The V
O
X (X = Ti, H) atoms are allowed to move, and the rest are kept frozen to the geometry optimized at 0 K. This approach is a good compromise between the quality of the results and the computational cost and has proven to be reliable in the interpretation of experimental spectra in supported vanadia catalyst models; see, for instance, refs 17 and 33. Intensities have not been calculated. Although the calculated frequencies suffer from the well-known problems of the method used, namely, harmonic approximation and the use of a pure DFT functional, the results shown provide a rough guide of the evolution of some spectral features, rather than fine numbers to be directly compared with experiment. A titania-supported sample was prepared as described previously,34 with a total coverage of 1.2 V atoms per nm2 of TiO2 support (named 1.2 VTi), close to the coverage modeled in the calculations, 0.84 V atoms per nm2 of TiO2. Raman spectra were acquired with a Renishaw system 1000 Raman system with an ultra-long distance 20 objective using a

514.5 nm excitation line and 1 mW power on the sample. The sample was exposed to dry and humid air (0.03 atm H2O partial pressure) in a Linkam TS-1500 in situ hot stage. The spectra were acquired during stepwise heating of the sample from room temperature up to 673 K.

3. RESULTS 3.1. Structures of Vanadium Oxide Species. Monomeric surface vanadium oxide species are found to be stabilized compared to dimeric species. Monomers evolve to different 4- and 5-fold coordinated structures after addition of successive water molecules to the low-loading vanadia
titania models. Scheme 1 presents the vanadium conformations that appear during molecular dynamics at 300, 500, and 800 K. Five structures correspond to 4-fold coordinated (tetrahedral, denoted as Tx) vanadium oxide species. They interact with the (001) titania anatase surface through a different number of bridging V—O—Ti bonds. A decrease of bridging V—O—Ti bonds is compensated by simultaneous appearance of new V—OH bonds. The OVO3 pyramid contains a terminal oxygen and three bridging oxygen atoms binding to surface titanium atoms (Scheme 1, T1). The former constitutes a vanadyl VdO group pointing out of the surface plane. Structure T2 is an exclusive 4-fold coordinated species with a bridging V—(OH)—Ti bond. Its geometry resembles that of OVO3 pyramid with a hydrogen atom adsorbed on the bridging V—O—Ti oxygen atom. The hydroxo-monovanadate OV(OH)O2 binds to the titania surface through two bridging V—O—Ti bonds (Scheme 1, T3). One vanadyl VdO bond and one V
OH hydroxyl group point out of the surface. The V—O— Ti bonds form one plane perpendicular to the plane formed by OdV—OH groups. The OV(OH)2O pyramid contains terminal oxygen, two hydroxyl groups, and one bridging oxygen atom binding to the surface titanium atom (Scheme 1, T4). The next hydroxyl group added to the monovanadate species leads to the formation of an “umbrella”-like structure, where the vanadyl VdO group together with two V—OH groups are situated at the same plane. Structure T5 is an example of totally hydrated monovanadate conformation (precursor molecule). It contains one vanadyl VdO group and three V—OH groups. The OV(OH)3 24135

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Figure 1. Statistical distribution of surface species after water addition to low coverage vanadia/titania model, calculated at 300, 500, and 800 K: 4-coordinated monomeric surface VOx species (Tx, top) and 5-coordinated monomeric surface VOx species (Px, bottom). The % represents the number of times a given structure appears in the MD run with respect to the total number of structures counted.

monomer does not present any bridging V—O—Ti bonds and is stabilized only by hydrogen bonds with the titania surface. Four structures correspond to 5-fold coordinated (pentahedral, denoted as Px) vanadium oxide species. Concerning the geometry, they can be divided into two groups. The first group exhibits a square pyramidal geometry, where four oxygen atoms bind to surface titanium atoms (Scheme 1, P1, P2). A vanadyl VdO bond points out of the surface plane in both structures. Structure P1 is free of hydroxyl groups, while P2 contains two bridging V—(OH)—Ti hydroxyl groups. The second group exhibits a trigonal bipyramidal geometry (Scheme 1, P3, P4). Structure P3 contains a vanadyl VdO bond pointing out of the surface and one hydroxylated group forming a V—(OH)—Ti bond. Monovanadate species P4 is an exclusive 5-fold coordinated conformation presenting a terminal oxygen (VdO) and hydroxyl group (V—OH) pointing outward from the surface, similar to the T3 structure. Besides this, P4 shows one V—(OH)—Ti bond. 3.2. Evolution of Surface Vanadium Species during Hydration Process. The V
OH bond formation and its transformation between monomeric species were studied by successive addition of water molecules to the models. It reflects the hydration process of a dehydrated vanadia
titania surface. The structures appearing in the molecular dynamics (MD) runs were

classified as Tx or Px by visual inspection, and their appearance frequency, i.e., the number of times a given species appears during the runs, was counted. Thus, a predominant species appears many times in a run, while a minority species presence is scarce. Figure 1 exhibits the statistical presence of each vanadium monomer moiety as a function of water molecules present in the model at given temperatures. The dehydrated OVO3 pyramid (T1) is predominant under completely dehydrated conditions; it becomes minor even at low water content. At 300 and 500 K, the T1 predominance decreases from 100 to ∼15%, whereas it sharply drops from 100 to ∼10% at 800 K (Figure 1). The OV(OH)O2 hydroxo-monovanadate moiety (T3) appears simultaneously with the disappearance of OVO3 monovanadate (T1). T3 is a predominant structure in the presence of water for all the temperatures considered (Figure 1). The T3 structure exhibits a maximum of ∼40% presence for two water molecules at 300 and 500 K, and it reaches 50% for three water molecules at 800 K. T2 and T4 hydroxylated species appear after a second water molecule addition to the dynamic models. These never go beyond ∼15% at 300 and 500 K, reaching 20% at 800 K. A totally hydrated T5 conformation appears only at a higher hydration level but remains a minority. The OV(OH)3 molecule or monomer is present in the system after addition of a fourth water 24136

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Figure 2. Hydrolysis process of monomeric surface VOx species after addition of four H2O molecules to the low vanadia/titania model. Molecular dynamics calculated at 800 K in the femtoseconds (fs) time scale.

Table 1. Total Energy Optimized at 0 K for the 1VOx/TiO2 Model with Different Numbers of Water Molecules (All Energies in eV)

a

number of

ETotal

H2O molecules

(relative)

0a


2155.94 (0.00)

0

T1; T1

1a


2172.83 (0.00)


2.67

T1; T1

1 2a


2172.27 (0.56)
2189.29 (0.00)


2.11
2.45

T3; T3 T3; T3 T1; T3

Eads per H2O molecule

VOx structure

2


2187.43 (1.86)


1.52

3a


2205.41 (0.00)


2.27

T3; T3

3


2203.40 (2.01)


1.60

T1; P4

4a


2219.74 (0.00)


1.73

T3; T3

4


2218.78 (0.96)


1.49

T2; P4

8a


2278.07 (0.00)


1.05

T3; T3

8 8


2277.68 (0.39)
2277.67 (0.40)


0.997
0.996

T3; T4 T4; T4

See models in Figure 3.

molecule regardless of the temperature used for the molecular dynamics calculation (Figure 1). Interconversion between surface species is observed at all temperatures and is faster at 800 K. The Px vanadium oxide species is much less frequent than the Tx species (Figure 1). An interesting finding observed in some runs at 800 K is the leaching of vanadia units from the surface as a consequence of the VOx species solvation process. Figure 2 depicts the process in the femtosecond (fs) time scale. Adsorption of four dissociated water molecules mainly on the V2O5 unit leads to the formation of a totally hydrated OV(OH)3 unit which finally leaves the surface. The process is as follows. The evolution begins from a dimeric vanadate species in a trigonal bipyramidal geometry (marked by the arrow) exhibiting several hydroxyl groups. The initial structure, not showing any vanadyl group, evolves to restore a vanadyl VdO bond by displacing one hydrogen atom from the V—OH group to the Ti—O group (Figure 2, 200 fs). The V—(OH)— Ti and V—(OH)—V bonds break and transform into two new V—OH hydroxyl groups. The vanadate moiety still exhibits a 5-fold coordinated structure bound to the surface by one bridging V—O—Ti bond. Subsequently, the bridging V—O—Ti bond breaks and liberates a 4-fold coordinated OV(OH)3 monomer (Figure 2, 400 fs). The monomer first interacts with the titania surface by hydrogen bonds and then becomes mobile and leaves the surface (Figure 2, 480 fs). Leaching is observed experimentally and will be discussed below.

Figure 3. Most stable models of 4-coordinated structures of the low loading vanadia/titania catalyst after addition of water molecules; optimized at 0 K. Water molecules are marked by black circles.

3.3. Stability of Vanadia
Titania Models under Hydrated Conditions. The stability of vanadia
titania models depends on

the conformation of surface vanadia species and on the distribution of the OH groups on the surface. The calculations of total energy were done for selected structures obtained from MD runs at 800, 500, and 300 K. Table 1 summarizes the total energy data of the models, the adsorption energy of each water molecule added, and the type of monovanadate structure. The results collected in Table 1 correspond to the most favorable conformations found as well as to the alternative configurations, albeit energetically less favorable. The stable conformations of the monovanadates as a function of the number of water molecules are presented in Figure 3. Structures T1 and T3 of vanadate monomers predominate in the most favorable conformations. The T1 moiety corresponds to the dehydrated OVO3 pyramid. It is the main species in dehydrated and “slightly” hydrated conditions (i.e., 1 H2O molecule adsorbed on the titania surface). The T3 structure is a hydroxo-monovanadate, OV(OH)O2. It appears as the most favorable conformation under hydrated conditions (i.e., >2 H2O) (Figure 3). Although the energetically favorable models show exclusively T1 and T3 monovanadate structures, other structures such as T2, T4, and P4 are also possible especially for high water content (Table 1). Water dissociates essentially up to three H2O molecules; at higher water content or pressure, it adsorbs in its molecular form (black circle in Figure 3). The support plays a role in water dissociation and in the hydroxyl group formation, which are observed on both titania and vanadia. 24137

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Figure 4. Adsorption energy per H2O molecule related to the most stable models of 4-coordinated structures of the low loading vanadia/ titania catalyst; optimized at 0 K.

Surface hydroxyl groups and molecular water form hydrogenbonding networks, denser as hydration progresses (Figure 3). A significant distortion in the first atomic layer of the support is observed as a consequence of the presence of hydroxyl and vanadate units. Figure 4 depicts the changes of the adsorption energy per water molecule corresponding to the most stable conformations of the monovanadates (Table 1 and Figure 3). A relationship between the adsorption energy per H2O molecule and the number of H2O molecules per unit cell shows nearly linear dependence. The adsorption of the first water molecule causes a decrease of total energy by
2.67 eV. Further adsorption of H2O molecules on the surface is still energetically favorable, but the energetic gain is lower with each successive molecule. In order to estimate the relative stability between the chemisorbed species, we use an atomistic thermodynamics approximation, as described and used with success in refs 5, 15, and 16. We consider the vanadia/titania system in contact with a gaseous H2O reservoir. From the electronic energy, the free energy of the water/vanadia/titania interface, under known thermodynamic conditions, may be estimated. Basically, it consists of 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 ZPE correction) in the gas phase is ΔGðH2 OÞ ¼ EðH2 OÞ
ððΔHG
TΔSG ðTÞÞ þ RT lnðp=pÞÞ

ð1Þ 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 Gaussian 03 code31 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.29 Using the above-mentioned formalism, the free energy of reaction for the formation of the hydrated structures under equilibrium conditions can then be expressed as ΔGn ¼ Eðtitania
V 2 O5
ð8
nÞH2 OÞ
nΔGðH2 OÞ
Eðtitania
V 2 O5 Þ

ð2Þ

Figure 5. Free energy ΔG of the complexes on the 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).31 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 = 180 K, followed by the structure containing three water molecules.

In this approximation, we consider that the energies of the hydroxylated species are independent of the degree of hydration of the titania surface. Figure 5 shows the free energy ΔG of the complexes on the 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).31 From our set of structures, the most stable isomeric forms are retained to draw the phase diagram. At the abovementioned conditions, the structure with eight water molecules is the most stable until T = 180 K, followed by the structure containing three water molecules. From the thermodynamic approach, one concludes that the most stable species for a wide range of temperatures is T3, OV(OH)O2. The completely dehydrated structure T1 would only be obtained at high temperatures, low water pressures. 3.4. Vibrational Spectrum. The calculated harmonic frequencies for the species T1 and T3 are displayed in Figure 6. Note that no correction has been applied to the numbers so they overestimate experimental values in the case of hydroxyl and vanadyl vibrations. In the case of V—O—Ti vibrations, their assignment is still under debate, since their calculated wavenumber strongly depends on the method used.35 Besides, such vibrations are not necessarily active in IR or Raman. The present results are thus intended to give an indication of the trends to be expected in experiments rather than to provide fine numbers. The VOH stretching mode is found to be a narrow band at 3848 cm
1. Hydrogen bonding to other surface groups causes this band to widen and redshift toward lower wavenumbers. Interestingly, this redshift is also observed for the VdO band: isolated groups show a narrow band at 1071 cm
1 which shifts to 1009 cm
1 upon interaction with surface hydroxyl groups. This redshift is observed in Raman spectra upon hydration of the sample.36 Two groups of bands are found for the V—O—Ti vibrations: symmetric stretching at 930 cm
1 and asymmetric at 793 cm
1 for T3 and 853 and 728 cm
1 for T1. The hydroxyl bending appears at 738 cm
1. Raman experiments on low coverage vanadia/titania 24138

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Figure 6. Calculated harmonic vibrational spectra for the most stable species, T1 and T3. The average values for each range are given in cm
1. No correction has been applied to the numbers shown. νVOH, VOH stretching; νVdO, VdO stretching; νs, symmetric V—O—Ti stretching; νas, asymmetric V—O—Ti stretching; δV—OH, V—OH bending.

Figure 7. (left) Representative Raman spectra of a catalyst containing 1.2 V atoms per nm2 of TiO2 (1.2 VTi) in dry (green trace) and humid (blue trace) air flow at 473 K. (right) Raman vanadyl vibration mode vs temperature for 1.2 VTi in dry (green trace) and humid (blue trace) air.

catalysts36 attribute VdO vibrations for dehydrated systems at 1028 cm
1, decreasing until 1006 cm
1 in the presence of water vapor. Higher coverage shows an additional band at 920 cm
1 assigned in that work to V—O—V bands due to polymerization. Our calculations do not allow confirmation of such assignment, since no polymeric species are found to be stable in the low coverage models used. In order to experimentally assess the calculated frequencies, in situ Raman spectra of 1.2 VTi were acquired in dry and humid air at increasing temperatures. Figure 7 (left) illustrates representative Raman spectra at 473 K. The Raman band near 1020
1022 cm
1 corresponds to the vibrational mode of the VdO bond of molecularly dispersed vanadium oxide on titania, whose frequency depends on coverage and exposure to humidity.37 Exposure to humidity is known to redshift the VdO mode.36 Figure 7 (right) confirms the trend predicted for calculated harmonic frequencies (Figure 6) of the VdO mode upon exposure to water; i.e., the band shifts to higher frequency as the temperature

increases and as the environment goes from humid to dry air stream.

4. DISCUSSION 4.1. Structure of Vanadium Oxide Species. The structure of the vanadium oxide species during the hydration process depends on two main factors: (i) the degree of hydration and (ii) the temperature. Raman and FT-IR studies of vanadia-supported systems under hydrated/dehydrated conditions have pointed out these two factors.38
40 Nevertheless, the experimental studies did not specify the most favorable structures and the type of interaction between the functionalities present at the surface of the support. The molecular dynamics calculations confirm that monomer is a preferred structure of the surface vanadium oxide species at low loading on titania. Splitting of dimeric V2O5 units deposited on the (001) titania anatase surface already proceeds during molecular dynamics of the dehydrated surface at high 24139

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The Journal of Physical Chemistry C temperature; the two new monomeric units remain very close to each other. Such splitting and subsequent monomer formation process is promoted by the progressive addition of water molecules. Previous theoretical studies of low loading vanadia
titania systems with DFT calculation15 and semiempirical EH method16 suggested that monomeric and dimeric species can coexist on the TiO2 surface. The former are stable only when they are isolated. The present work shows that the majority of the surface VOx species obtained during molecular dynamics are monomers. The tendency to form monomers by V
O
V bond breaking is independent of the degree of hydration. The presence of water facilitates the separation process. Hydroxylated monovanadates evolve preferably into 4-fold coordinated structures independently of the hydration degree (Scheme 1, Tx). The preference of VOx and VOxHy species for preserving the tetrahedral coordination is confirmed by theoretical15
17,19,20 and experimental41,42 studies. Among the 4-fold coordinated moieties, structures T1 and T3 are the preferred conformations. Whereas the T1 structure represents a totally dehydrated species, T3 corresponds to a hydrated one. They exhibit a complementary population distribution trend: the OV(OH)O2 hydroxo-monovanadate (T3) appears concomitantly to OVO3 monovanadate (T1) depletion. The most energetically favorable systems during progressive water addition contain exclusively these two predominant conformations. It means that these two species are the most stable ones under dehydrated and hydrated conditions, respectively. They have also been proposed as stable vanadium-supported conformations in other theoretical calculation studies.15
17,19,20,43 The hydroxo-monovanadate (T3) species possesses a hydroxyl group directly bound to the vanadium atom. The formation of a V
OH bond has been confirmed by FT-IR for VOx/TiO211,39,44 and VOx/SiO240,42,45,46 catalysts. Both OV(OH)O2 (T3) and OV(OH)2O (T4) moieties exhibit V
OH groups formed by hydrolysis of V
O
Ti bridging bonds. Theoretical calculations reveal higher stability and higher probability of existence for the former than for the latter species under mild hydration conditions. The “umbrella”-like structure mostly appears at higher water content (above four H2O molecules) or at higher temperature (800 K). These conditions facilitate the hydrolysis of the second V
O
Ti bridging bond. The coexistence probability of T3 and T4 conformations increases at higher water content (eight molecules), since they exhibit similar energy values (Table 1) and the interconversion seems feasible in the presence of high water content. The occurrence of T2 and Px monomers in the dynamics of the hydration process is much lower than the other ones. The common feature of these 4- and 5-fold coordinated structures is the bridging V
(OH)
Ti bond, which easily hydrolyzes, forming V
OH or Ti
OH groups. In this context, T2 and Px play a role of intermediate structures during system evolution to more stable configurations. These reactive intermediates are essential for gaining an energetic optimum by surface moieties during hydration, but they are difficult to characterize by experimental techniques. Although they appear in molecular dynamic calculations at different degrees of hydration and at different temperatures, they do not typically appear as optimized structures (Table 1). Pentahedral species are found to be less stable than tetrahedral in the case of low vanadia coverage. Busca et al. have reported the presence of pentacoordinated vanadia species upon interaction with water or methanol molecules using IR spectroscopy.44 In the present work, these seem to play a role as an intermediate in the leaching of OV(OH)3 moieties (Figure 2). In line with the

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literature,44 DFT calculations indicate that these appear to be potentially reactive in the methanol oxidation reaction.47 In that study, the energy involved for the formation of formaldehyde via pyramidal methoxy (∼T1) species and via pentahedral (P) methoxy species is found to be similar, although the relative stability of the two intermediates is clearly in favor of T1. This means that the T1 methoxides will cover the surface and be detected, but it does not exclude the possibility to form overcoordinated species upon exposure to methanol. Such species, being less stable, would react faster in a parallel path. 4.2. Dynamics of the Surface Species. The surface of vanadia
titania catalysts at low coverage reveals dynamic changes during the hydration process, which result from intrinsic structural properties of vanadium oxide species and mobility of hydrogen atoms. Increase of temperature intensifies the interaction between surface moieties and their interconversion. Monovanadates with V+5 oxidation state show a preference to possess a terminal oxygen (VdO), also seen in other materials such as silica and zeolites.17,48
50 Surface species adsorbed on titania with exclusively V—OH hydroxyl groups evolve to a VdO vanadyl-containing configuration. Such evolution proceeds via a hydrogen atom transfer from the V—OH group to the Ti—O—Ti site or other surface moieties such as Ti—OH, V—O—Ti, and VdO (Figure 2). The hydrogen transfer mechanism is observed to take place to preserve tetrahedral coordination around vanadium. At high water content, the OV(OH)3 species stay near the titania surface and interact by hydrogen bonds, before leaving the surface (Figure 2). It should be noted that the higher the temperature, the higher their mobility over the surface support. The changes in the surface vanadium oxide upon progressive hydrolysis are described by in situ Raman spectroscopy.36,38,51 Raman spectral features of dehydrated and hydrated species differ significantly. The VdO Raman band shifts from 1027 cm
1 for the dehydrated surface to 1019 cm
1 for the partially hydrated surface until 1000 cm
1 for the highly hydrated surface. The redshift of the VdO band results from hydrogen bonding interaction between the vanadyl bond and surface hydroxyl group (V—OH, Ti—OH) or water molecule; the solvated vanadium oxide species possess a structure closer to that of the hydrated vanadate species. Structural transformations to T3 (OV(OH)O2), T4 (OV(OH)2O) until T5 (OV(OH)3) moieties occur frequently in the presence of water. Most of them are reversible at lower content of water. Dihydroxo-monovanadate (T4), “umbrella”-like structure, exhibits a high instability and easily transforms to hydroxo-monovanadate (T3) by hydrogen atom transfer to the other surface features or to solvated vanadate (T5) by hydrolysis of the V—O —Ti bond. The transitions depend on the degree of hydration and on the temperature of the system. Monovanadate moieties transform to reach the most stable configuration (T3) at low water content. The calculations show that hydrolysis becomes increasingly important with water content, as expected, illustrating the rationale of how this process is occurring. Hydrogen transfer is a key performance in the reversible processes of interconversion between different surface vanadium structures. At lower temperatures, the degree of hydration induces the structural changes on the surface, whereas, at elevated temperatures, hydrogen transfer drives the transformations. The mobility of hydrogen is invoked for the separation of polymeric vanadate species and the evolution between different monovanadate structures. The mobility process depends on the facility to achieve the energetically favorable structures. The transfer of hydrogen atoms between vanadate and titania surface decreases after the 24140

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The Journal of Physical Chemistry C system reaches the most favorable monovanadate structures, T1 and T3. Hydrogen atoms move equivalently from Ti—OH and V—OH surface hydroxyl groups to titanyl (TidO) or vanadyl (VdO) surface sites. If the transfer occurs to the bridging V—O—V and V—O—Ti bonds, it leads to bond breaking. The hydrogen transfer easily occurs between V—OH and VdO groups. The hydrogen mobility is also responsible for water molecule formation. Molecular water mostly forms on Ti—OH sites by hydrogen migration from Ti—OH and V—OH groups. Water molecules are stabilized at the catalyst surface by hydrogen bonds with different surface features. The process of water creation is reversible, and H2O molecule can undergo dissociation at elevated temperatures. The hydrogen atom mobility was postulated on the basis of FT-IR bands of VO—H modes recorded at different temperatures.39 The large decrease in the intensity of the VO—H band together with the redshift in the band position with temperature were correlated to the presence of surface structural changes based on hydrogen atom transfer. Although the hydrogen mobility was discussed as a reason of the observed changes in the FT-IR band position, it was mainly assigned to the higher vanadia loading catalysts. The mobility of hydrogen was rather excluded for the lower loading catalysts where predominately monomeric vanadia structures existed, since the hydrogen atoms were primarily associated with the titania support.39 The molecular dynamic calculations presented in this work confirm the possibility of the free hydrogen transfer between all surface features for the low vanadia loading systems. 4.3. Mechanism of Vanadia
Titania Surface Hydration. Totally dehydrated vanadia supported on the (001) titania anatase surface is energetically unfavorable, and it can only be achieved at high temperatures, >800K. The system exhibits a high affinity for moisture, since the adsorption of a water molecule on the surface causes a decrease of total energy by
2.67 eV. Water molecules adsorb dissociatively, forming two hydroxyl groups on the surface Ti V and O II sites, hydrogen-bonded to the vanadate monomers. The monovanadate OVO 3 (T1) structure appears exclusively in the most energetically favorable conformations until a water molecule adsorbs. The assumption of partial surface dehydration is more plausible than that of complete dehydration. Additional water adsorption is still favorable, although the energy gain is lower (Table 1). Dissociative adsorption is predominant for the first three water molecules and involves both titania support and supported vanadia monomers. Hydroxo-monovanadate OV(OH)O2 (T3) structure predominates in the stable systems. It means that the next stage of the hydration process is formation of a V
OH bond. The hydroxyl group adsorption or hydrogen atom transfer generates a V
OH moiety. Further hydration involves molecular water at the surface. The generation of a surface water layer increases the probability of hydrolysis of surface vanadium oxide species. The molecular dynamics calculations exhibit the formation of fully solvated OV(OH)3 species above the catalyst surface after the addition of four water molecules. The hydrolysis of bridging V
O
Ti bonds intensifies after consecutive addition of H2O molecules to the system. Although the energetically favorable conformation exhibits T3 species even at high water content (8 H2O), the other systems with mixed T3/T4 and exclusively T4 species exhibit total energy values close to the preferred conformation. The presence of the dihydroxo-monovanadate “umbrella”-like moieties on the hydrated titania surface results from the hydrolysis process. These species can be easily solvated and leave the surface as fully solvated T5 OV(OH)3 species. The transformation of surface vanadium oxide species to solvated

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OV(OH)3 finishes the hydration process of the vanadia
titania surface. In the whole hydration process, a network of hydrogen bonds forms. This network stabilizes the system and plays an important role in the hydrogen atom transfer between the surface species. The enhancement of water adsorption and dissociation by the formation of hydrogen bonds was reported for the hydration process of the (101) TiO2 surface25 and the (100) MgO surface.52 Spectroscopic techniques recorded the changes in the surface vanadium oxide species during the hydration process. Nevertheless, they are still unable to distinguish each specific monovanadate species formed during hydrolysis. Spectroscopy exclusively confirms the presence of fully dehydrated OVO3 or solvated OV(OH)3 species and appearance of V
OH bonds.

5. CONCLUSION The molecular dynamics calculations of vanadia
titania catalyst hydration reveal the importance of the process conditions on the global changes of surface species. The degree of hydration and the temperature strongly affect the formation of hydroxylated vanadate species and transformation between them. Tetrahedral monomers are preferred surface vanadium oxide species at low vanadia loading on titania. Splitting of dimeric V2O5 units deposited on the (001) titania anatase surface proceeds readily on the dehydrated surface at high temperature. Hydration is a thermodynamically favorable process, and hydroxyl-containing species rapidly form in the presence of low water content. The most stable species is OV(OH)O2; the OVO3 pyramid would only be stable in severe dehydrated conditions at high temperatures (>800 K). Water dissociative adsorption on titania support and supported vanadia species is preferred at low hydration levels, whereas higher hydration levels favor V
O
Ti hydrolysis and the formation of molecular water. As the hydration degree increases, leaching of OV(OH)3 species is observed, which is consistent with known experimental data.51 Interconversion between different surface species occurs easily and reversibly depending on the temperature and water content. While temperature tends to increase the rate of surface species mobility, hydration has several effects. First, water is directly involved in hydrolysis processes breaking surface bonds: it stabilizes monomers with respect to dimers and breaks V
O
Ti bonds. Second, water provides stable surface hydroxyl groups such as V
OH and Ti
OH. The calculations indicate that such coordination has a effect on the vanadyl Raman mode vibration, which experimentally confirms the relevance of temperature and humidity. Finally, the formation of a hydrogen network is observed. This network facilitates rapid hydrogen (proton) transfers between surface species favoring their interconversion. The hydrogen transfer is not directed and protons move equally from Ti
OH and V
OH surface hydroxyl groups to the other surface features. The presence of water at high temperatures is important to understand titania-supported catalysts that run in the presence of water vapor or in reactions that generate water. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figure showing the periodic model of the low loading vanadia/titania catalyst. This material is available free of charge via the Internet at http://pubs.acs.org.

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The Journal of Physical Chemistry C

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses #

Cardiff University, School of Chemistry, Park Place Cardiff CF10 3AT United Kingdom. E-mail: LewandowskaA@cardiff.ac.uk.

’ ACKNOWLEDGMENT The authors thank Spanish Ministry of Science and Innovation project CTQ2008-04261/PPQ and ESF COST Action D36. 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. thanks HPC resources from GENCI- CINES/IDRIS (Grant x2010082131), the CCRE-DSI of Universite 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. This work was supported by EULANEST, and Spanish MICINN (PIM2010EEU-00138). ’ REFERENCES (1) Wachs, I. E. Catal. Today 2005, 100, 79. (2) Grzybowska-Swierkosz, B. Appl. Catal., A 1997, 157, 263. (3) Bond, G. C. Appl. Catal., A 1997, 157, 91. (4) Dias, C. R.; Portela, M. F.; Bond, G. C. J. Catal. 1995, 157, 344. (5) Busca, G.; Elmi, A. S.; Forzatti, P. J. Phys. Chem. 1987, 91, 5263. (6) Forzatti, P.; Tronconi, E.; Busca, G.; Tittarelli, P. Catal. Today 1987, 1, 209. (7) Forzatti, P.; Tronconi, E.; Elmi, A. S.; Busca, G. Appl. Catal., A 1997, 157, 387. (8) Zhu, J.; Anderson, S. L. T. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3629. (9) Miki, J.; Osada, Y.; Konoshi, T.; Tachibana, Y.; Shikada, T. Appl. Catal., A 1996, 137, 93. (10) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Appl. Catal., B 1998, 18, 1. (11) Topsøe, N.-Y.; Topsøe, H.; Dumesic, J. A. J. Catal. 1995, 151, 226. (12) Topsøe, N.-Y.; Dumesic, J. A.; Topsøe, H. J. Catal. 1995, 151, 241. (13) Ovchinnikova, E. V.; Andrushkevich, T. V.; Shadrina, L. A. React. Kinet. Catal. Lett. 2004, 82, 191. (14) Zhu, J.; Lars, S.; Andersson, T. Appl. Catal. 1989, 53, 251. (15) Vittadini, A.; Selloni, A. J. Phys. Chem. B 2004, 108, 7337. (16) Ferreira, M. L.; Volpe, M. J. Mol. Catal. A: Chem. 2000, 164, 281. (17) Islam, M. M.; Costa, D.; Calatayud, M.; Tielens, F. J. Phys. Chem. C 2009, 113, 10740. (18) Grybos, R.; Witko, M. J. Phys. Chem. C 2007, 111, 4216. (19) Calatayud, M.; Mguig, B.; Minot, C. Theor. Chem. Acc. 2005, 114, 29. (20) Calatayud, M.; Minot, C. Top. Catal. 2006, 40, 17. (21) Gonzalez-Navarrete, P.; Gracia, L.; Calatayud, M.; Andres, J. J. Comput. Chem. 2010, 31, 2493. (22) Gracia, L.; Gonzalez-Navarrete, P.; Calatayud, M.; Andres, J. Catal. Today 2008, 139, 214. (23) Lazzeri, M.; Vittadini, A.; Selloni, A. Phys. Rev. B 2001, 63, 155409. Lazzeri, M.; Vittadini, A.; Selloni, A. Phys. Rev. B 2002, 65, 119901. (24) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Gr€atzel, M. Phys. Rev. Lett. 1998, 81, 2954. (25) Arrouvel, C.; Digne, M.; Breysse, M.; Toulhoat, H.; Raybaud, P. J. Catal. 2004, 222, 152. (26) Dzwigaj, S.; Arrouvel, C.; Breysse, M.; Geantet, C.; Inoue, S.; Toulhoat, H.; Raybaud, P. J. Catal. 2005, 236, 245. (27) Calatayud, M.; Mguig, B.; Minot, C. Surf. Sci. 2003, 526, 297.

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